In accordance with conventional practice in the electrolyser art, electrolytic cells may be connected in series in a common housing, with the anodes of one cell being electrically in series with the cathodes of the prior cell and mounted on the opposite sides of a common structural member. In this way, the cathodes of one cell are in series with the anodes of the next adjacent cell in the electrolyser and mounted on a common structural member, and the anodes of the cell are in series with the cathodes of the prior cell in the electrolyser. Such a configuration is called a bipolar configuration.
An electrolyser is an assembly of electrolytic cells in a bipolar configuration. The common structural member is called a bipolar unit or bipolar electrode. This includes the backplate, the anodes of one cell in the electrolyser and the cathodes of the next adjacent cell. The electrolytic cell provided by the anode of one bipolar electrode, facing the cathodes of the adjacent bipolar electrode, so that electrolysis of the electrolyte may be carried out, is called a "bipolar cell".
Bipolar electrolysers provide economy of materials of construction and plant space. However, to take advantage of the apparent economies of bipolar electrolysers, it is necessary for current density to be as high as possible. When electrolysis is carried out at high current density, it is necessary that there is minimal electrical resistance between elements of the bipolar electrode. It is also important that seepage, of electrolyte, between elements of the bipole, is prevented.
Chlorine is produced in vast quantities by a variety of salt electrolysis processes. There are three principal processes operated; mercury cell; diaphragm cell and membrane cell. To a lesser extent, chlorine is also produced from electrolysis of hydrochloric acid although the technology has lagged behind electrolysis of salt solution.
In conventional electrolysis processes based on hydrochloric acid, the hydrochloric acid (typically 22 wt % HCI) is fed into the cells in two separate circuits, a catholyte and an anolyte circuit. During electrolysis the concentration is reduced to approximately 17%. The electrolyser is bipolar, with pairs of electrodes arranged like a filter press. A diaphragm separates the anode compartment from cathode to prevent mixing of the gaseous products. Both anode and cathode are graphite and the diaphragm is PVC fabric. Chlorine dissolved in the anolyte diffuses through the diaphragm and is reduced at the cathode causing a loss of 2-2.5% of the theoretical current yield. Hydrogen ions are also transmitted through the diaphragm under influence of the applied field and maintain the overall process in balance, as hydrogen ions are reduced at the cathode to hydrogen. Each electrolyser consists typically of 30-36 individual cells formed from vertical graphite plates connected in series and separated by a PVC diaphragm. This process is operated on a large scale as a convenient method of recycling chlorine in organic synthesis where hydrogen chloride is produced as a byproduct. It is believed that this equipment is unsuited to the relatively small scale requirements of the water industry.
The chlorination of potable water using gaseous chlorine was first experimentally employed in 1896 and is still the prime method of disinfection today. Since the early 1970's, due to the potential dangers of transporting and storing large volumes of gaseous chlorine, an alternative in situ method of generation have been developed. This process involves electrolytic conversion of salt solution to chlorine in solution as sodium hypochlorite. Although successfully adopted by many water authorities, there are a number of disadvantages including:
does not generate gaseous chlorine; PA1 involves complex equipment to prepare brine solution; PA1 pH control necessary; PA1 complex chemistry involved; PA1 includes dosing salt into water being disinfected; and] PA1 only partial conversion of salt possible. PA1 poor dimensional stability; PA1 massive constriction necessary because of low mechanical strength; PA1 high energy consumption; PA1 complex design to accommodate variations in inter-electrode gap due to high wear rate; PA1 chlorine contains hydrogen and carbon dioxide; PA1 hydrogen contains chlorine; and PA1 difficult to manufacture a cell of filter press form.
The major technological step forward in chlorine cell technology in the last 30 years has been the adoption of coated titanium electrodes (anodes). Prior to this discovery anodes were made of graphite and used exclusively for more than 60 years. Since 1970 all chlorine plants operating on saturated brine have been converted to titanium anodes. However, a similar adoption of titanium anodes has not occurred in hydrochloric acid electrolysis because of two main problems.
The first problem concerns the corrosivity of hydrochloric acid to titanium and the operational constraints of noble metal coatings at low pH values. For example, manufacturers of titanium normally state that titanium is only moderately resistant to hydrochloric acid, quoting a corrosion rate of 4.4 mm/year at 20% acid concentration (normal electrolysis concentration). Also, noble metal anode coatings are thought to wear more rapidly when the pH drops below 4.
The second problem concerns the anode of electrical connection. Industrial electrochemical cells can be connected in mono or bipolar configuration, but in hydrochloric acid, titanium is not viable as a cathode because of corrosion via titanium hydride formation. Also, it is not possible to join other metals to titanium by conventional welding methods because of the formation of brittle intermetallic compounds.
In early chlor-alkali bipolar electrolysers, flow of electricity through the bipolar structure was enhanced by providing metal to metal contact between the titanium anode and steel cathode, by explosive bonding. However, it was soon found that hydrogen generated on the steel cathode surface migrated through the steel towards the titanium. This resulted in the formation of titanium hydride at the interface between the steel and titanium.
In the simplest form of bipolar electrode, titanium is coated on one side only; the reverse uncoated side being the cathode. As in chlor-akali, sea water electrolysers suffer from similar problems but at neutral pH values, hydride formation is not as severe. In older designs utilising rotating bipolar titanium electrodes, the formation of titanium hydride on the cathode surface is relatively slow. The main problem is mechanical; because titanium hydride has a lower bulk density than titanium, the structure gradually deforms, as one side becomes less dense than the other. The life of such electrodes in sea water electrolysis is 40% of those that do not use the reverse side as a cathode.
The problems associated with, and the mechanism of formation of titanium hydride, have been studied extensively. Hydrogen is unique in its ability to penetrate many of the metals from its gaseous state. This penetrative ability is enhanced by ionization or dissociation into atomic form. In addition, metals can become more susceptible to hydrogen penetration by physical form or temperature range. Some metals are non-occluders. Others react to form salts (the alkali metals of Group 1 and 2). Others form gaseous products (arsine). The transition metals are inert or form hydrides via endothermic or exothermic reactions. Titanium is in the group of transition metals, with the largest capacity to absorb hydrogen. This group absorbs hydrogen accompanied by reaction without loss of metallic characteristics. However, the accompanying 15% volume increase, in the case of titanium, can cause mechanical deformation.
Titanium is widely used in hydrogen containing environments under conditions where hydrogen could be evolved on titanium and, consequently, its susceptibility has been widely studied. Generally, it is believed that under conditions of neutral pH, ambient temperature and low salinity (sea water composition), hydride formation is confined only to titanium surfaces.
As a result of the foregoing problems with titanium, such as hydride formation, the conventional wisdom in relation to hydrochloric acid electrolysis has therefore been that graphite is the only viable electrode material, although it has several disadvantages, including:
Therefore, there is a need for practical bipolar titanium electrodes for the electrolysis of hydrochloric acid. Further, there is a need for an improved method of performing the electrolysis of hydrochloric acid.